DEVITRIFICATION OF GLASS. 247 



occurrence. — Natural glass is an abundant constituent of the effusive rocks. 

 It is especially prevalent in the more acid ones, but is not confined to them, 

 being not infrequently abundant in the intermediate rocks, such as basalts. 

 A lava or tuff may be almost wholly composed ot glass, or glass may con- 

 stitute but a small part of the background. There are thus all g-radations 

 between completely crystalline rocks and glassy rocks. Of the more recent 

 effusive rocks glass not infrequently composes a large part of the flows. 

 An instance is Obsidian Cliff, in the Yellowstone National Park. But in 

 proportion as lavas are old, glass is less and less likely to be found, and in 

 the more ancient lavas is ordinarily absent. The explanation of this 

 absence is devitrification after solidification. 



Evidence that devitrification takes place. That Certain l - Ocks 110W wholly COlXLpOSed 



of minerals were once glasses is shown by the preservation in perfection of 

 the flow structures and very delicate trichitic, perlitic, spherulitic, and other 

 textures characteristic of glass. 



scale of devitrification. — It is also certain that the process of devitrification 

 has taken place in nature on a great scale. As evidence of this may be 

 cited the well-known American instances of devitrified glass in the original 

 Huronian district, described by Williams," the aporhyolite of South Moun- 

 tain, Pennsylvania, described by Williams and Bascom, 6 the metarhyolites 

 of the Fox River Valley of Wisconsin, described by Weidman," and the 

 devitrified glasses of the Crystal Falls district of Michigan, described by 

 Clements/ In the papers of these authors many other instances of 

 devitrification are cited, including European instances. 



Not only does devitrification of natural glass take place, but under 

 proper conditions artifical glass devitrifies in a similar manner. Well- 

 known cases of the devitrification of artificial glass under conditions of 

 weathering are those of the buried ancient glasses of Nineveh and of Rome. 



a Williams, G. H., Notes on the microscopical characters of rocks from the Sudbury mining 

 district, Canada: Ann. Kept. Geol. and Nat. Hist, Survey of Canada, vol. 5, Pt. F, Appendix 1, 1890- 

 1891, pp. 74-82. 



b Williams, G. H., The volcanic rocks of South Mountain, in Pennsylvania and Maryland: Am. 

 Jour. Sci., 3d ser., vol. 44, 1892, pp. 486-490. 



Bascom, Miss Florence, The ancient volcanic rocks of South Mountain, Pennsylvania: Bull. U. S. 

 Geol. Survey No. 136, 1896, pp. 42-61. 



cWeidman, Samuel, A contribution to the geology of the pre-Cambrian igneous rocks of the Fox 

 River Valley, Wisconsin: Bull. Wisconsin Geol. and Nat. Hist. Survey No. 3, 1898, pp. 4-31. 



^Clements, J. Morgan, and Smyth, H. L., The Crystal Falls iron-bearing district of Michigan: 

 Mon. U. S. Geol. Surv., vol. 36, 1899, pp. 87, 101-103, 138. 



